If you run a BSD type of OS like i.e. MacOS you must use "-r" instead of "-d@" to convert from unix timestamp.

Update 2015-01-11: Insert additional grep for "iTunesMetadata.plist" to speed up. Hint: If you use "Payload.*PkgInfo" instead of "iTunesMetadata.plist" you will get a date near by the build date of the app instead of the purchase date.